This invention relates generally to methods for routing telephone calls and more particularly to methods for routing telephone calls to Subscribers of different service providers within a common Numbering Plan Area (NPA).
As is known in the art, telephone calls are routed from a calling Subscriber to a called Subscriber through a network of switches. Subscribers connected to a common switch, or End Office (EO) are assigned a unique directory number, NXX-XXXX, where "N" refers to any digit except 0 or 1 and "X" refers to any one of 10 digits. Thus, for example, all telephone Subscribers in a small town are connected to a single, "local" switch (or End Office (EO)) and all telephones directly served, or connected to the switch, or End Office (EO) have a directory number which uses as the first three digits thereof, a switch, or End Office (EO) code, NXX. These three digit prefixes are sometimes referred to as an "exchange or office code". The last four digits, XXXX, are referred to as the SUBSCRIBER's line code. Larger towns may have two, or more larger switches, each with several different three digit prefix switch or exchange codes, NXXs.
As is also known in the art, the telephone system divides the United States into "area codes"; more technically referred to as Numbering Plan Area (NPA) codes. When a call is made from one "area code" to another "area code", the three digit Numbering Plan Area code, NPA, prefix must be supplied to the called Subscriber's directory number (DN). Thus, in effect, each telephone Subscriber is associated with a unique ten digit directory number; NPA-NXX-XXXX.
As is also known in the art, the United States has been subdivided into Local Access and Transport Areas (LATAs), each LATA includes a contiguous sub set of the Numbering Plan Areas (NPAs). Calls between LATAs are routed via inter-exchange carriers, IXCs. Such inter-LATA calls need only be routed to the appropriate IXC carrier based on the calls originator (i.e., the calling Subscriber's) Inter Exchange Carrier (IXC) access code (either dialed or pre-subscribed). Intra-LATA calls are routed based on the directory number, which is of the format NPA-NXX-XXXX. The NPA-NXX (Area code-exchange code) prefix is used to route the call to the serving, local switch, or End Office (EO). From there the local switch routes the call to the Subscriber's line which is designated by the last four digits, XXXX.
As is also known in the art, in the past, in most regions of the United States, a Local Service Provider (LSP) (i.e., a regional telephone company) had exclusive routing of calls within a Local Access and Transport Area (LATA). The regional telephone company therefore controlled the local switches and assigned the connected subscriber a directory number (DN). Today, each LATA may be served by other, competing Local Service Providers (LSPs). Therefore, if a Subscriber wishes to be served by a different Local Service Provider of the same Local Access and Transport Area (LATA) the Subscriber would have to change the previously assigned directory number (DN), since directory numbers are assigned to a switch in blocks and are used as a means for routing calls to the switch serving those local directory numbers as described above.
If a Subscriber wished to keep the previously assigned directory number, calls to the Subscriber had to be routed, via the switch which owns the directory number block, and from there to the Subscriber's new switch using an unpublished directory number for that routing purpose. Such method, however, ties up the new switch during the period of time the call is routed through it.
One technique suggested to enable the Subscriber to maintain the previously assigned directory number (DN) after changing to a new Local Service Provider (LSP) in the same Local Access and Transport Area (LATA) is to translate the previously assigned directory number (DN) to a Location Routing Number (LRN) code which is any valid switch code, NXX, assigned to the switch which presently serves the Subscriber. The Numbering Plan Area (NPA) code would be the same as the NPA of the directory number (DN) because the Subscriber has not physically moved to another area. The NPA-LRN code is passed along with the directory number (DN), NPA-NXX-XXXX to a tandem as well as terminating switches end offices (EOs). Thus, a two numbers are used: NPA-LRN plus NPA-NXX-XXXX. The NPA-LRN is used for routing and the NPA-NXX-XXXX is used in the terminating switch to locate the Subscriber's line and any associated features (i.e. special services, such as call waiting, call forwarding, etc.) which are encoded as "classmarks". Such technique requires that the existing trunk signaling protocols be modified, especially multi-frequency and Dial Pulse, which do not support passing two numbers associated with the called Subscriber.